The present invention relates to a magnetic head for efficiently, recording and reproducing high frequency signals and which is suitable for use in systems dealing with such high frequency signals, such as a high-definition video tape recorder, digital video tape recorder or the like.
Conventionally, in devices for recording and reproducing high frequency signals such as video tape recorders, etc., ferrite material with little high frequency loss has been generally employed as a magnetic material for the video heads. In recent years, however, systems dealing with signals in a still wider band region, such as a high-definition video tape recorder, digital video tape recorder or the like, are being vigorously developed, and recording media therefor are being altered from iron oxide media to high coercive media such as metal powder media, metal evaporated media, etc. in the trend towards high density construction for recording such a large amount of information. Meanwhile, the ferrite head has its maximum magnetic flux density at about 5000 gauss, and for efficiently reproducing short-wave length signals; it is required to use narrow gaps, and thus, in the high coercive media having Hc higher than 1000 oersted, the ferrite core at the gap forward end portion is saturated, making it impossible to effect perfect recording. Accordingly, development of a magnetic head employing metallic magnetic materials such as sendust and amorphous magnetic alloys having high maximum flux density has been attempted, but the metallic magnetic material in a bulk state as employed has a large high frequency loss due to eddy current, and can hardly be used for the system as described above. For this, use of the metallic magnetic material formed into a thin film has been investigated in order to suppress such a loss as far as possible, and by way of example, it is intended to cope with high frequencies by constituting the main magnetic circuit as a laminated structure of the metallic magnetic thin films and insulative thin films.
In a high-definition video tape recorder, digital video tape recorder, etc., the recording signal band region thereof extends in the range of 30 to 60 MHz, and as the core material for the magnetic head, a material having a high initial permeability in such a high frequency band region is required.
In the diagram of FIG. 5, there is shown a frequency characteristic of initial permeability where direction of anisotropy of an amorphous magnetic thin film with respect to a measuring direction is altered, in a laminated structure of CoNb Ta Zr amorphous magnetic thin film and SiO.sub.2 film. The thickness of the magnetic thin film per one layer is set to be 4 .mu.m by taking the eddy current loss into consideration, and SiO.sub.2 film thickness between layers is set at 0.2 .mu.m for lamination in 5 layers.
In FIG. 5, a curve (1) represents a random oriented lamination film, in which, although the eddy current loss is improved by the lamination effect, the high frequency characteristic thereof is restricted by Snoek's limit due to ferromagnetic resonance, and the initial permeability at the high frequency band region above 30 MHz falls below 500. Therefore, such a random oriented magnetic film, if employed as a head core, can not correspond to the high frequency system as described earlier.
On the other hand, the initial permeability characteristic of a multi-layer film in which amorphous magnetic thin films having uniaxial anisotropy are laminated, with directions of easy axes thereof aligned, is as shown in FIG. 5, and upon measurement in the direction of the easy axis, an extremely low initial permeability characteristic is observed over the entire frequency band region as represented by a curve (2), while upon measurement in the direction of the hard axis, high initial permeability is maintained up to high frequencies as represented by a curve (3), with a value above 1000 even at 60 MHz.
However, in the case where a magnetic head having a comparatively large coil winding opening, as in a video head or the like, is constituted by the magnetic core having anisotropy in one direction, the easy axis direction is to be contained in its magnetic path, with a large reduction in head efficiency. Moreover, constitution of all magnetic paths in the hard axis direction is very difficult from the viewpoint of head manufacturing methods.